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Workshops >> Heap Leach and Waste Rock Facility Closure Workshop
Integrating Geomorphology, Reestablishment of Vegetation Communities, Cover Performance Monitoring, and ET Cover Modeling for the Bazza Waste Rock Facility Closure Design, Barrick Goldstrike Mines William Schafer, Schafer Limited LLC, Bozeman, MT Johnny Zhan, Barrick Goldstrike Mines Inc., Elko , NV Ken Myers, The Mines Group, Inc., Reno , NV Mike Milczarek, GeoSystems Analysis, Inc., Tucson , AZ Steve Viert, Cedar Creek Associates, Ft. Collins, CO Joe Giraudo, The Mines Group, Inc., Reno , NV Ron Espell, The Mines Group, Inc., Reno , NV Abstract: Barrick Goldstrike Mining Inc. (BGMI) operates the Goldstrike Mine, which is located near Elko in north central Nevada , USA . Goldstrike is a large open pit and underground Carlin Trend gold mining operation. The mine consists of the Betze Post Pit, the Bazza WRF, the AA Leach Pad (reclaimed), the North Block Tailings Facility, the Rodeo and Meikle Underground Mines, a Roaster and an Autoclave Facility (Figure 1). Closure planning at BGMI is a continuous process of concept development, design, construction, performance monitoring, evaluation and refinement. The concepts involved in operation and closure of the Bazza WRF are to characterize acid rock drainage (ARD) potential and segregate potentially acid generating (PAG) material, shape the pile to a geomorphically stable form that blends with existing topography, integrate channels into the hillslope design, place an ET soil cover, and rapidly establish vegetation that evolves into a permanent diverse plant community. An ET cover is designed to store excess meteoric water that occurs between October and March, and releasing the stored water by evapotranspiration from April through September. This presentation describes each of the primary components of the closure design for the Bazza facility.
Waste Rock Dump Rehabilitation techniques in the Goldfields of Western Australia; issues and future strategies to achieve Mine Closure Steven Rusbridge, Placer Dome, Western Australia Abstract: Mining has been an integral part of the Western Australian Goldfields since gold was first discovered in the region in 1892. By the 1900’s the easily won gold had been found and big companies were formed to exploit the deeper deposits. Surface disturbances were relatively limited, except for localized clear felling of trees to supply fuel and mine support timbers. The development of improved processing techniques and lower mining costs in the mid 1980’s saw many large open pit gold mining operations commence. This led to a rapid increase in the number of large waste landforms being constructed in the region. Initially, very little was understood about rehabilitation techniques in this environment. Initial methods used were those borrowed from other regions. In 1996 the Department of Minerals and Energy published its “Guidelines for Mining in Arid Environments”. These guidelines contained some excellent information on how to develop and rehabilitate minesites in the region. The downside was that they became “quasi-regulation” and often led to a “one size fits all” approach being adopted by companies. It soon became apparent that these guidelines were not suitable in many cases, however by now they had worked their way into legally binding lease conditions and a large number of sites had adopted them. Because of the climatic conditions, problems of landform design often may not manifest until many years after construction, and changes that are made to “fix” one problem, may create another. Research and trials continue to build knowledge and understanding of rehabilitation processes. In particular, erosion modeling software (such as WEPP SIBERIA) show promise in helping develop appropriate design. This has led to the use of this type of modeling for assessment of “left field” approaches such as ‘accelerated erosion’ of highly erodable waste landforms back into pits. This presentation will examine these issues by looking at the good, the bad and the ugly of rehabilitation in the Goldfields and some current trials and research being adopted by Placer Dome in an attempt to address these issues.
The Gold Quarry Slide: Event and Geotechnical Investigation Jeff White, Newmont Mining Corporation, Reno , NV Abstract: On the evening of 5 February 2005 , operations personnel at Newmont’s Gold Quarry Mine north of Carlin, NV observed surface cracks at the crest of the North Waste Rock Facility. A slope failure over the next few hours resulted in the displacement of over 10 million tons of earthen material some of which buried a portion of State Route 766. No people were injured in the event nor were there impacts to nearby Maggie Creek . In coordination with the Nevada Department of Transportation and neighboring landusers, alternative transportation routes were quickly established. Newmont made notification of the event and road closure to a large group of ‘stakeholders’ and routinely communicated with key individuals and entities. In cooperation with the Nevada Division of Environmental Protection (NDEP), Newmont developed an initial action plan for stabilization of the slide area and excavation to expose the roadway. Newmont moved swiftly to develop and implement an investigation plan to determine the cause(s) of the slide. NDEP established the Gold Quarry Slide Oversight Panel to oversee the investigation of the slide and associated immediate and long-term actions. Newmont routinely reports investigation status and remedial action progress to the Panel. The investigation approach is comprised of several elements including:
When completed, the reports resulting from the investigation, Gold Quarry North Waste Facility Investigation and Slide Mechanics and Gold Quarry North Waste Facility Mitigation and Long-Term Stability Plan, will be reviewed by the Panel. The stability plan will be approved by NDEP prior to implementation. This paper will discuss the slide event and the geotechnical investigation process.
Preliminary Status Report On Molycorp Goathill North Trenches, Questa , New Mexico Patrick Walsh, Virginia T. McLemore , Kelly M. Donahue, Luiza A. F. Gutierrez, Samuel Tachie-Menson, Heather R. Shannon, and G. Ward Wilson New Mexico Bureau of Geology, Socorro, New Mexico and Department of Mining Engineering, University of British Columbia Abstract:Rock pile characterization has become important in mine closure in order to prevent acid rock drainage and to alleviate long-term stability concerns. Rarely do rock pile characterization methods allow for examination and sampling of the undisturbed interior of large rock piles in-situ. The regrading of the Goathill North (GHN) rock pile at the Molycorp Questa mine, New Mexico provided a unique opportunity to examine and sample the undisturbed, interior of a large rock pile through the construction of trenches cut into the rock pile as earth-moving progressed. Weekly during the regrading of the GHN rock pile, contractors excavated a trench to allow for sampling of rock pile material. Trenches typically had four benches, which were 1.5 m wide and did not exceed 1.2 m in height, to give an overall slope of 1.4 horizontal to 1.0 vertical within the trench. Each trench extended for a length sufficient to explore site conditions, maintain the regraded 2:1 slope, and ensure personnel safety. Once excavated, trench walls and benches were surveyed using a differential global positioning system. For every trench, maps and logs of each bench were created to describe the different “mine soil” units, including the thickness, dip and extent of the units. Units were defined based on color, grain size, stratigraphic position, and other physical properties that could be determined in the field. Units were correlated between benches and to both sides of each trench, and several units were correlated downward through the series of successively excavated trenches. The field sampling crew began sampling within each of the identified units after the unit boundaries were identified and mapped. The following in situ measurements were taken along either the horizontal or vertical surfaces of each exposed bench and along the base of the trench: sand cone (density), tensiometer (matric suction), gravimetric moisture content, grain size, infiltration, and nuclear gauge measurements (density, moisture content). Gravimetric water content samples were collected at the locations selected for the measurement of matric suction and infiltration tests. Samples were collected from each defined unit for geochemical, geotechnical (including shear box tests, slake durability tests), biological, and electron microprobe analyses. Channel sampling for pyrite reserve modeling was performed in short 1.5-m long horizontal slots using a rock hammer to chip material to be placed into a sample bag. Additional material from selected layers was collected for potential weathering-cell tests in the future. Typically, paste pH increases with distance from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile.
Treatment of Acid Drainage at the Source Jack Adams, University of Utah , Salt Lake City, UT Abstract: When solid-phase pyrite in rock is exposed to oxygenated water acid drainage is typically formed by the oxidation of pyrite (FeS 2) containing reduced iron and sulfur species. As a result of this oxidation, the concentration of both reduced iron and acid in the drainage water increases significantly. Additionally, the simultaneous presence of reduced iron and oxygen promotes the growth of acidophilic, autotrophic iron- and sulfur-oxidizing bacteria such as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and Sulfolobus acidocaldarius. The oxidation of Fe 2+ serves as the rate-limiting step in biologically-mediated acid production. In most waste rock piles with acid drainage, T. ferrooxidans and other iron oxidizers often out-compete other indigenous heterotrophic bacteria species present due to low organic levels. However, under the right environmental conditions, indigenous and inoculated heterotrophic microbes can increase in concentration at faster rates than autotrophic bacteria. Additionally, in the absence of oxygen, certain heterotrophic microbes are capable of coupling organic carbon oxidation to ferric iron reduction. Significant reductions in acidophilic, iron- and sulfur-oxidizing bacteria were obtained in column tests by the addition of organic materials. Iron oxidation was inhibited as the treatment reduced dissolved oxygen levels with concomitant increase in indigenous and inoculated non-iron-oxidizing heterotrophic bacteria. Furthermore, as the system became anoxic, the much of the oxidized iron species was consumed resulting in a pH increase. As the applied organics and microbial materials decrease the populations of acidophilic bacteria it leads to the dominance of heterotrophic bacteria in biofilms that coat rock surfaces. This has secondary beneficial effects of metal sulfide precipitation and lowering the redox environment, further inhibiting growth of iron- and sulfur-oxidizing species. Results suggest that biologically and chemically mediated acid drainage can be reversed by dispersion of organic treatment materials and microbes into the subsurface.
Innovative Contracting and Management in an Industry-Sponsored Research Consortia - Impact Weathering on Rock Pile Stability – Questa , NM Terry Chatwin, University of Utah , Salt Lake City, UT Abstract: In implementing a research project on the “Impact of Weathering on Rock Pile Stability” Molycorp with the assistance of the University of Utah developed a unique contracting and management model. This model is designed to create an environment for scientific discovery and investigator independence while instilling the commercial focus of being “on time” and “on budget.” To this end, a modified earned-value contract was created with both fixed-price and cost-reimbursable components included. One third of the contract is associated with tangible deliverables or milestones such as reports that represent key information to be shared between co-investigators. Each of these milestones has a fixed budget based on principal investigator’s burden salary. As this work is completed milestones payments are paid. The remaining two thirds of the budget is paid on a cost-reimbursable basis, and is associated with students’ salaries, travel expenses, and laboratory charges. As an incentive to universities, Molycorp prepays on a quarterly basis on projected cost-reimbursable charges. This prepayment shortens the payment cycle, while still retaining close budget accountability. The program progress and expenditures are tracked monthly through the use of a simplified earned-value reporting system. The advantages and uses of these management tools are presented. Other unique features of the contract and organization structure as well as the rationale behind this arrangement are discussed in this paper.
Field evaluation of waste rock geochemical reactivity, and its role in providing comprehensive dump characterization Jeff Parshley, Steven Day and Rob Bowel, SRK Consulting, Reno, NV Abstract: A common concern in mine closure when considering geochemical characterization is how representative a sample set is of overall geochemical conditions within a given unit such as a waste rock dump. Traditional approaches require either a minimum number of samples proportional to tonnage or volume, or a variable density sampling grid, either regular or irregular in distribution. Although both approaches have gained acceptance there are limitations in both that compromise the overall integrity of a sampling campaign. Selecting a random number of samples proportional to overall waste rock mass or volume has the inherent risk that some units may not be sampled, particularly small units that could exert a greater control over drainage water quality than a more dominant rock type that is chemically less reactive. Development of a grid approach may be biased towards material placed at a particular time or material segregated on the dump. In addition the full assessment of validity cannot often be made until after sample analysis is complete, potentially resulting in additional expense and delay required to re-sample underrepresented units to provide adequate coverage. Such issues prompted the authors to develop a phased strategy for sampling waste rock that utilizes geological and mineralogical mapping with semi-quantitative geochemical field reactivity testing. This provides a rapid characterization of not only the geological characteristics of the units comprising the dump but also allows the evaluation of variation in geochemical reactivity within each of the major rock units. Thus, by use of these field methods an adequate laboratory sample set can be collected that fully characteristics the geological components of the dump and the range of variable geochemical reactivity observed for each of the geological components. The methodology will be illustrated by recent waste rock evaluation studies conducted in Idaho ( Atlanta ) and Nevada (Tonopah and Robinson).
Tailings at the Small Scale: New Science May Lead to New Strategies Lesley A. Warren, School of Geography and Earth Sciences, McMaster University, Hamilton, Ontario, Canada Abstract: Tailings management strategies grapple with the long-term reality of tailings discharges and the ability to reliably predict effluent quality. Unanticipated changes in tailings water quality emerge, likely bacterially associated, that can be problematic to plan for or constrain effectively once they occur. Whilst these issues scale to bulk system, geochemically important processes driving these outcomes occur at the microscale, commonly at sediment water interfaces or within redox gradients, where bacteria often thrive and potentially influence geochemistry through their metabolic activity. New microscale technologies permit high resolution, investigation of geochemical behaviour within these reactive zones, whilst culture-independent methodologies provide the ability to characterize the bacteria involved. My group seeks to characterize key processes and their controls, whether geochemical or bacterial, that influence metal and acid generation in mine environments. I will present an overview of two of our projects at the Falconbridge Onaping mine, ( Onaping , ON ), evaluating the potential role of bacteria in tailings metal dynamics as well as acid generation within the tailings lake. We use a suite of new techniques spanning geochemistry (microelectrodes), molecular biology (characterization of bacteria) and molecular level microscopy (e.g. X-ray Absorption Spectroscopy, XAS), coupled to intensive, high-resolution field sampling (both temporal and spatial) in order to evaluate system dynamics. The key strength of our approach is both the spatial resolution at which we investigate (often μm to <mm), and the frequency with which we sample (2-3 times over a 24 hour period) in order to capture important processes as well as constrain their relative controls and impact. Bacteria significantly impact metal and acidity behaviour in these tailings systems, and they do so under conditions that differ from those predicted by equilibrium geochemical models. Given the widespread occurrence of bacteria across mine environments, the implications of our results extend beyond this specific site. Our approach provides new insight into bacterially driven processes contributing to bulk system characteristics which are not currently well constrained. Such information will help in the development of more effective strategies for long-term tailings management or perhaps improved leaching.
Waste rock backfill of open pits in Nevada : Design, optimization, and modeling considerations Brent Johnson , Kenneth Carroll, Water Management Consultants Inc , Denver, CO and Tucson, AZ Abstract: Pit lakes that develop in open pit mines after mining is stopped are potential sources of contamination to groundwater, surface water, and the surrounding ecological system. Evapoconcentration in pit lakes alters the water chemistry and can result in undesirable concentrations. In Nevada , some pit lakes discharge to groundwater while others behave as hydraulic sinks, resulting in no discharge to the groundwater system. Most pit lakes must be closed and managed over the long-term to minimize the potential exposure to humans and wildlife habitats. This presentation discusses considerations for backfilling open pits that would normally function as hydraulic sinks if a pit lake was allowed to develop. Backfilling with waste rock is one method for mitigating pit lake environmental issues. However, while fully-backfilled pits result in no exposure to these habitats, there may be discharge to groundwater because the hydraulic sink is eliminated. Partial backfill designs may be optimized to an elevation near the pit lake level, which eliminates the lake but not the passive hydraulic sink. Optimized backfilling is becoming an increasingly attractive closure option of mine operators and State regulators. For the mine operator, pit backfill could mean potentially less costly haulage routes, and a location to dispose of reactive (e.g., acid-generating) waste rock. For regulators, pit backfill, if placed to optimized elevations, results in a long-term passive hydraulic sink (i.e., no discharge of impacted water to groundwater), and no pit lake exposed to humans or wildlife habitats. This presentation provides a summary of pit backfill closure design concepts for pit lakes that would normally function as hydraulic sinks. The designs are described in terms of their potential to provide hydrologic and geochemical containment/isolation of pit lake/pore water solutions. Finally, this presentation describes some tools to design and simulate the performance of optimized pit backfill closures.
Drain-Down of a Heap Leach Pad M.D. Fredlund, SoilVision, Saskatoon , Saskatchewan, Applied Soil Water Technologies, LLC, Sparks , Nevada Abstract:The number of mining companies extracting metals from ore through the heap leaching process has increased in the past few years. The infiltration technology has improved and the process has been largly proven effective. In spite of the general effectiveness of this process the inner workings of heap leach pads remains largely a “black box”. Calculation of unsaturated seepage through a heap leach pad is numerically challenging due to the nonlinearity of unsaturated flow and the highly irregular 3D geometry encountered in most heap leach operations. This presentation examines the application of the SVFlux finite element numerical software to the analysis of drain-down for a large heap-leach operation. The data availability, upscaling of soil properties and the convergence issues are examined to determine the suitability of application of numerical software to heap leach operations.
From Numerical Modeling to Practice: Cover Performance Data for Five Heap Leach Pads Maritz Rykaart & Steve Boyce, SRK Consulting ( Canada ) Inc., Vancouver, British Columbia, Canada Abstract: A recent international review of cover design and construction practices, which included more than 180 sites, confirmed two primary design practices for soil covers; (1) precedence based design, i.e. designing covers by simply mimicking what has been done at other locations, and (2) modeling based design, i.e. using a numerical modeling tool to develop the cover design parameters. Furthermore, un-calibrated numerical modeling emerged as being the most common form of modeling based design. This paper will present five case studies of recently reclaimed heap leach pads in Nevada for which the cover design has been based on such un-calibrated numerical modeling. The covers were subsequently constructed according to the modeled design parameters, and their performance has been monitored regularly since. The authors will present the cover performance data as compared to the original modeled predictions, and illustrate a number of interesting and important trends with respect to the ability to predict cover performance with the tools that are currently available. There are also a number of important lessons that can be learned from these case studies, especially with respect to the selection of cover design standards, and appropriate mitigation measures.
Vegetation as the Primary Regulator of Infiltration – Vegetation’s Role & Case Histories Fred Marinelli, PhD, PE and Barry Carlson, PE, Telesto Solutions, Inc., Fort Collins , Colorado Abstract: Infiltration through soil covers at reclaimed heap leach facilities is an area of increasing interest to the mining community. This is because after reclamation, infiltration of meteoric water tends to be the only source of recharge, and this ultimately controls the long-term drainage flow rate from the facility. Numerous evaluations have been performed to estimate the mean annual infiltration into reclaimed heaps. These have included instrumentation of heap covers and simulation of unsaturated flow processes using numerical models. Heap facility investigations suggest that plant viability and root biomass distribution is the primary control on infiltration, especially long term. Plants are viewed as a dynamic control on infiltration that, over the long term, self-adjust intercept available soil moisture and provide salinity control. If there is no leakage through the base liners, reclaimed heaps can be viewed as large-scale lysimeters that (after draindown) can provide a field measurement of the infiltration rate. It is our opinion that when properly screened and qualified, drainage flow rates from reclaimed heaps are the most reliable means of assessing infiltration, and once steady-state rates are achieved, represents a direct correlation. There currently exist numerous reclaimed heaps in the State of Nevada that are routinely monitored for drainage. Based on data compiled by NDEP, site visits, and Telesto corporate experience, we have begun an effort to correlate measured heap drainage rates with the types, densities, and relative health of plant communities existing on the heap slopes. Although this effort is ongoing, preliminary results suggest that reasonable correlations exist between infiltration and the viability of plant communities. Other issues such as cover thickness, soil texture, and slope angle are important, but secondary in their respective impact on long term infiltration.
Semi-Passive Bioreactors at the Leviathan Mine Timothy K. Tsukamoto and Glenn C. Miller, Center for Environmental Sciences and Engineering, University of Nevada , Reno Abstract: The use of sulfate-reducing bioreactors to treat acid mine drainage has advantages over current active treatment technologies due to the passive to semi-passive nature of the treatment as well as the lower volumes of sludge produced, which both contribute to lower operational costs. In 2002, a semi-passive bioreactor was constructed at the Leviathan Mine, Alpine County California. This remote site is located at approximately 7000 ft elevation on the eastern slope of the Sierra Nevada Mountains . Alcohols are utilized by sulfate-reducing bacteria to reduce sulfate to sulfide and precipitate metals as metal-sulfides. Because alcohols do not freeze under normal site conditions, this carbon and energy source can be gravity fed to supply the bacteria with specific concentrations of reducing equivalents throughout the year. A rock matrix with large pore spaces is utilized in conjunction with a flushing mechanism to reduce the chance of plugging and short circuiting within the bioreactor. In addition, the majority of the metals are removed outside of the bioreactor. Treated water, laden with sulfide is mixed with untreated water in a settling pond where the metals are removed. Water, essentially free of metals, is then passed through the bioreactor where sulfate-reduction and sulfide generation occurs. This system is less active than conventional lime treatment and can operate for longer periods of time without replacement of the matrix when compared to traditional passive bioreactors.
Oxidation of Acid Mine Drainage with an Innovative Rotating Cylinder Treatment System (RCTS) Timothy K. Tsukamoto, Ionic Water Technologies, Boise, ID Abstract: Impacted acid mine drainage typically contains elevated concentrations of dissolved ferrous iron. The oxidation of ferrous iron to ferric iron is a common component to most lime treatment systems because ferric iron is precipitated from solution at a lower pH than ferrous iron. This oxidation is typically accomplished by pumping air with compressors and mixing the air, lime and water with agitation mixers in large tanks. Although this method of treatment is effective, it requires significant power and a large amount of space to house the reaction tanks. The Rotating Cylinder Treatment System (RCTS) “patent pending” utilizes shallow trough like cells (that contain the water being treated) and rotating cylinders to transfer oxygen and agitate the water. This system has been tested on multiple sites in Nevada and California . When compared with conventional systems it requires less power, and less space and is more efficient at mixing lime. |
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